Clock-doubled CPUs: A PC with an Intel 486DX2 processor runs internally at 50 MHz. What speed does its external bus (front-side bus) operate at?

Introduction / Context:Many classic processors (e.g., 486DX2) used internal multipliers to run the CPU core faster than the external front-side bus. Recognizing the relationship between internal clock and bus clock is essential when evaluating memory throughput and expansion card timing.

Given Data / Assumptions:

• CPU is a 486DX2, which uses a 2x internal clock multiplier.
• Internal core frequency is 50 MHz.
• We are asked for the external bus frequency.

Concept / Approach:

The 486DX2 multiplies the external bus frequency by 2 to obtain the internal CPU frequency. Therefore, bus_clock * 2 = internal_clock. Rearranging gives bus_clock = internal_clock / 2. With a 50 MHz core, the bus runs at 25 MHz.

Step-by-Step Solution:

Verification / Alternative check:

Historical specs list 486DX2-50 as 25 MHz FSB with 2x multiplier. Similarly, a 486DX2-66 uses 33 MHz FSB with 2x multiplier to reach 66 MHz internal.

Why Other Options Are Wrong:

• 10 MHz and 100 MHz: do not satisfy the 2x relation for a 50 MHz core.
• 50 MHz: that would be a DX (1x) or DX4 at different settings, not DX2.
• None of the above: incorrect because 25 MHz fits perfectly.

Common Pitfalls:

Confusing DX2 (2x) with DX4 (3x) parts; forgetting that memory and ISA bus derived timings follow the external bus, not the internal multiplier.

Final Answer:

25Mhz